Konrad Rykaczewski1, John Henry J. Scott1, Andrei G. Fedorov2
1. Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
2. G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332
Vapor condensation is an essential part of many technologies in energy generation, automotive engineering, heating, ventilation, and air conditioning, and thermal management. As a consequence, even a moderate improvement in the heat transfer rate during this phase change process could lead to considerable economic savings. Eighty years ago, Schmidt and coworkers1 demonstrated that the heat transfer rate during dropwise condensation is an order of magnitude higher than during filmwise condensation. Unfortunately, premature degradation of the surfaces and coatings needed for dropwise condensation has prevented the use of this process in practical applications2. Due to their ability to shed water, bioinspired superhydrophobic surfaces have recently generated a lot of interest and research effort in their application as promoters for dropwise condensation. Several groups have demonstrated that properly designed nano and/or micro structured superhydrophobic surfaces can maintain dropwise condensation3-5. Most of the research effort in this area has been focused on the design of the superhydrophobic surfaces themselves and characterization of their wetting behavior6,7, and only a few studies have paid attention to the condensation dynamics8-13. All of those studies focused on growth of drops with diameters ranging from ~10 µm to a few millimeters. However, imparting nanostructure to the surface results in a significant increase in the number of droplets with diameters below 10 µm14, and those droplets account for the majority of the heat transferred during dropwise condensation15-17. Because the droplets are so small and because the process is dynamic and occurs on complex topography, Environmental Scanning Electron Microscopy ESEM is the preferred method for imaging in detail the growth of droplets in the sub-10 µm regime. In contrast to other electron beam effects occurring during ESEM imaging such as radiation damage due to water radiolysis, electron beam-induced surface wettability modifications, dynamic liquid charging, topographic contrast, and biological sample damage, the evaporation of condensed drops has not been systematically studied. In this work17, we demonstrate that electron beam can cause significant heating and fast evaporation of condensed drops during imaging at high magnification necessary for observation of drop growth dynamics in the sub-10 µm regime. We characterize the electron-beam heating effects by observing the evaporation rates of condensed water droplets under different conditions in the ESEM. Further, we explain the observed experimental trends with a developed model of the process. We quantify the ESEM imaging magnification limit below which heating effects are negligible and use this insight to study individual droplet growth during water condensation on randomly stacked cupric hydroxide (Cu(OH)2) nanotube-based superhydrophobic surfaces.
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17. Rykaczewski, K., Scott, J.H.J., Fedorov, A.G., submitted, January 2011.